Chipmunk nailed it right off.... the organ blowers draw less current when the backpressure is high (the organ's not playing) and rises quite rapidly as more stops are used.
It has to do a lot with the design of the blower's impeller, certain designs place less of a load on the motor with higher back (or "static") pressure. The same organ blowers cannot be operated at all without being connected to the air distribution system or it will cause a severe overload. (I have heard of an instance where a 50hp organ blower was run with no air system connected, it took out a 2000 amp main breaker in no time flat.)
The starter I designed will also not allow the main (25hp) blower to restart until it has spun down to a stop, which can also cause an overload trip. (Time delay to accept start command after a stop command.)
Most organ blower motors don't rely on the airstream for cooling, they are either open frame or shaft mounted fan cooled.
A look though Grainger's catalog in the blower section will give a lot of insight as to different impeller designs and static pressure restrictions.
IIRC some water pumps follow the same principles.
